heat

For me, the epitome of stovetop alchemy is making caramel from table sugar. You start with refined sucrose, pure crystalline sweetness, put it in a pan by itself, and turn on the heat. When the sugar rises above 320°F/160°C, the solid crystals begin to melt together into a colorless syrup. Then another 10 or 20 degrees above that, the syrup begins to turn brown, emits a rich, mouth-watering aroma, and adds tart and savory and bitter to its original sweetness.

That's the magic of cooking front and center: from one odorless, colorless, simply sweet molecule, heat creates hundreds of
different molecules, some aromatic and some tasty and some colored.

How does heat turn sugar into caramel? Heat is a kind of energy that makes atoms and molecules move faster. In room-temperature table sugar, the sucrose molecules are jittery but standing in place, held still by the forces of attraction to their neighbors. As the sugar heats up in the pan, its molecules get more and more jittery, to the point that their jitters overcome the attractive forces and they can jump from one set of neighbors to another. The solid crystals thus become a free-flowing liquid. Then, as the temperature of the sugar molecules continues to rise, the force of their jittering and jumping becomes stronger than the forces holding their own atoms together. The molecules break apart into fragments, and the fragments slam into each other hard enough to form new molecules.

That's what I've thought for many years, along with most cooks and confectioners and carbohydrate chemists: heat melts sugar, and then begins to break it apart and create the delicious mixture we call caramel.

And we've all been wrong.

It turns out that, strictly speaking, sugar doesn't actually melt. And it can caramelize while it's still solid. So proved chemist Shelly Schmidt and her colleagues at the University of Illinois in studies published last year.

It's dismaying to think that so many could be so wrong for so long about such a basic ingredient and process! But it's also a rare opportunity to rethink the possibilities of the basic. Here's a plateful of possibilities; scroll down for more.

Professor Schmidt's group made their discovery when they tried to nail down the precise melting point of sucrose. The figures reported in the technical literature vary widely, and it wasn't clear why.

The melting point of a substance is the temperature at which it turns from a solid into a liquid while maintaining its chemical identity. When solid ice turns into liquid water, for example, the molecules of H2O move fast enough to escape the attractive forces of their neighbors, but they're still H2O. And it doesn't matter how fast the substance heats up: the melting point is the same. Ice melts at 32°F/0°C. Always.

After careful analysis, Professor Schmidt found that whenever sugar gets hot enough to turn from a solid into a liquid, some of its molecules are also breaking apart. So sucrose doesn't have a true melting point. Instead it has a range of temperatures in which its molecules are energetic enough to shake loose from their neighbors, and a range in which the molecules jitter themselves apart and form new ones. And these two ranges overlap. Whenever sugar gets hot enough to liquefy, it's also breaking down and turning into caramel. But it starts to break down even before it starts to liquefy. And the more that sugar breaks down while it's still solid, the lower the temperature at which it will liquefy.

When we make caramel standing at the stove, we use high heat to liquefy and then brown the sugar in a few minutes, and the liquefying temperature can be upwards of 380°F/190°C. But Professor Schmidt's group found that when they ramped up the heat slowly, over the course of an hour, so that significant chemical breakdown takes place before the solid structure gives way, the sugar liquefied at 290°F/145°C.

I made the caramelized sugars in these photos by putting crystals and cubes in my gas oven at around 250°F/125°C, shielding them with foil above and below to avoid temperature extremes from the cycling heating element, and leaving them there overnight and longer. In the large sugar crystals, which I got in a Chinese market, it's clear that breakdown and caramelization is fastest in the center. That may be because the center is where impurities get concentrated as the crystals are made, and the impurities then kickstart the breakdown process.

Caramel makers have long known that, as is true in most kinds of cooking, the key to caramelization is the combination of cooking temperature and cooking time. But the the temperatures have typically been very high, the times measured in minutes. Now we know that you can caramelize low and very slow and get something different. Sugar breakdown even occurs at ambient storage temperatures, though it takes months for the discoloration and flavor change to become noticeable. For a manufacturer this is undesirable deterioration. But for a cook in search of interesting ingredients, it could be desirable aging.

In a follow-up to her initial scientific reports, Professor Schmidt wrote in Manufacturing Confectioner that

from a practical point of view, caramelization can be thought of as browning of sucrose by applying heat for a length of time. Thus it may be possible to better control the caramelization reaction by identifying the time-temperature conditions that optimize the production of desirable caramel flavors compounds, while minimizing undesirable ones. Confectionery manufacturers and sugar artisans, armed with this new scientific knowledge, may be able to push their craft in unforeseeable directions.

In this month's Curious Cook column, I write about making iced tea and coffee by preparing them with cold water instead of hot. Aficionados differ on the relative merits of cold- and hot-brewed drinks. Cold brewing does extract a different balance of flavors compared to a standard hot brew. If you think of them as different drinks, then you can enjoy each for its particular qualities. I also describe recent studies of tea made with jamaica, the outer flower parts of a kind of hibiscus, and give recipes for jamaica and "mojito" teas from Maricel Presilla of Zafra and Cucharamama in Hoboken NJ.__________________

Mayer, F. et al. Sensory study of the character impact compounds of a coffee beverage. European Food Research & Technology 2000, 211:272-76.

EVEN in kitchens where fresh is king, the freezer remains a handy tool. There’s no easier way to deal with a bounty of meat from a big-box store or a butchering class or a C.S.A. share, or the haul from a fishing trip, or the unpredictable sighting of partridge and other rare birds in the Chinese market. In my house, the freezer is essential for drawing out the enjoyment of the prime mail-order meats that my mother sends for my birthday, and that arrive rock-hard under a block of dry ice.

Less handy, however, is the thawing process, which often requires planning a day or more ahead of the cooking. Food thaws slowly in the refrigerator, especially when kept in its plastic packaging, which is the method recommended by purveyors and the Department of Agriculture to minimize bacterial growth and the loss of juices. Thawing in cold water, 40 degrees or below, is safe and much faster — water transfers heat far more efficiently than air — but it can still take hours. I’ve never had much luck with the defrost setting on microwave ovens, which can start to cook one part of the food while the rest is still frozen.

Now there’s good news for last-minute cooks. It turns out that we can thaw frozen steaks and other compact cuts in as little as 10 minutes, without compromising their quality, and with very little effort. All you need is hot water.

This information comes, surprisingly, from research sponsored by the Department of Agriculture, though the methods aren’t yet officially recommended. The studies have been published in the Journal of Food Science and in Food Control.

At the U.S.D.A. labs in Beltsville, Md., Janet S. Eastridge and Brian C. Bowker test-thawed more than 200 one-inch-thick beef strip loin steaks in three different groups: some in a refrigerator at 37 to 40 degrees Fahrenheit, some in a constantly circulating water bath at 68 degrees, and some in a water bath at 102 degrees.

Air-thawing in the refrigerator took 18 to 20 hours, while the room-temperature water bath thawed the steaks in about 20 minutes, and the hot-summer-day bath in 11 minutes. These water-bath times are so short that any bacterial growth would remain within safe limits.

The water-thawed steaks actually leaked less juice than the air-thawed steaks. The researchers grilled the steaks, too, and found that all the thawed steaks lost about 26 percent of their original weight once cooked, while never-frozen steaks lost 21 percent. The study found no significant differences in tenderness between slow- and quick-thawed steaks.

Eleven minutes is pretty quick, but Brian A. Nummer and colleagues at Utah State University in Logan shaved away another couple of minutes by heating the water bath to 140 degrees, the standard temperature of steam tables in food service kitchens.

The Utah State group found that chicken breasts about a half-inch thick thawed in a little more than 3 minutes, and inch-thick breasts in less than 9 minutes. Although 140-degree water would eventually cook the chicken to medium-rare, they saw no signs of cooking. The quick-thawed breasts did lose slightly more juice than the refrigerator-thawed breasts, but when the chicken was grilled and served, a panel of 18 tasters was unable to tell them apart. And based on their mathematical modeling, the researchers concluded that any bacterial growth would remain well within safe limits.

So there’s no downside to quick-thawing steaks, chops, fillets and other relatively thin cuts in warm water right before cooking. Large roasts are a different story. They take long enough to thaw that there may be time for significant bacterial growth on their surfaces. Prompt cooking might well eliminate that problem, but until this has been studied, it’s safest to continue thawing roasts in the refrigerator or in water under 40 degrees.

Quick-thawing is easy to adopt in the home kitchen. But don’t expect your thaw times to match the lab times I’ve quoted unless you have an immersion circulator or another method to keep the water in motion and at a constant temperature. If the water is still, a cold zone develops around the food and insulates it from the remaining warm water. And without infusions of hot water or heat from a burner, the icy food cools the water bath.

Unless I’m in a rush, I’m happy to let the thawing proceed more slowly on its own while I take care of other tasks. I fill a large pot with 125-degree water from the tap, immerse the plastic-wrapped meat, weigh it down with a slotted spoon to keep it under water and stir the water occasionally. The water temperature drops, but stays above 100 degrees for a half-hour or so, depending on how much food is thawing.

Last week, I thawed 2-inch-thick filets mignons in an hour, whole squab in 40 minutes, a 1-pound whole fish in 20 minutes, and 1 ¼-inch-thick salmon fillets in 15 minutes. Thawing times can vary, depending on the volume, temperature and movement of the water as well as the food’s thickness and how it’s wrapped. (A lot of plastic swaddling interferes with heat transfer. It’s best to remove it and place the food in a thin resealable plastic bag, partly immersing it to force the air out before zipping it shut.)

So when you scan your larder to improvise a quick meal, don’t forget to look in the freezer. The makings of the main course may be just minutes away.

In this Wednesday's Curious Cook column I write about using a pot of hot water to thaw steaks, chicken breasts, and fish in minutes, rather than the few hours required in cold water or the many hours in a refrigerator. According to recent research funded by the USDA, rapid thawing can minimize the freeze-thaw damage to meat tissues, and taste panels couldn't tell the difference between meats that had been cooked after gradual or quick thawing.